The present invention relates generally to mechanical clutch systems which couple rotating shafts, and more particularly to the application of piezoelectric elements to rotary clutch systems.
The piezoelectric stack is an actuator used in many types of high-speed devices. This stack will extend or retract a small amount according to the voltage placed on its many separate plates of piezoelectric material. Its response is very fast, on the order of 100 microseconds to full extension or retraction. The second basic device is an adaptation of the sprag clutch, a familiar type of overrunning clutch.
Basically, the "piezostack" is capable of very fast response at high force, but the amount of displacement is so small as to render it unusable for a direct-clutch application; its elastic modulus when fully extended is not enough to support frictional forces high enough to transmit large forces quickly. Its extensions may be compounded in a pumping fashion to increase its displacement hydraulically, but this defeats much of the high speed advantage of the device.
The sprag clutch is a "passive" clutch, capable of engaging when the input shaft is rotating faster than the output shaft, and disengaging when the output shaft is turning faster than the input (i.e., overrunning it). For this reason it is known as an overrunning clutch. This clutch is very useful for certain types of high-torque applications, such as roll drives and motor starters. It is only controllable by the input or output shafts, accelerated or decelerated by conventional means. For high-torque power sources, this means overcoming a great deal of inertia, slowing the response time markedly.
A system which combines the piezoelectric stack with, clutch systems, such as the sprag clutch, would combine the fast response characteristics of piezoelectric elements with the high-torque engagement capabilities of the conventional clutch.
The task of applying a linear piezoelectric stack to activate a sprag clutch to achieve shaft coupling at high speeds and high torques is alleviated, to some extent, by U.S. Pat. No. 4,468,583 issued to Kenji Mori on Aug, 28, 1984, the disclosure of which is incorporated by reference. The Mori reference discloses a rotating actuator for converting electrical energy into rotating torque which includes annular piezoelectric elements alternately energized to continuously rotate a rotor.
While the device of the Mori reference is exemplary in the art, the use of such annular piezoelectric elements is described in Mori as generating a large torque at low rotation speeds. From the foregoing discussion, it is apparent that there currently exists the need to achieve shaft coupling at high speeds (3,000-25,000 rpm), as well as high torques. The present invention is intended to satisfy that need.